Some multi-band or other tactical radios operate in the high frequency (HF), very high frequency (VHF) (for satellite communications), and ultra high frequency (UHF) bands. The range of multi-band tactical radios can operate over about 2 through about 512 MHz frequency range. Next generation radios should cover about 2.0 to about 2,000 MHz (or higher) to accommodate high data rate waveforms and less crowded frequency bands. This high frequency transmit mode is governed by standards such as MIL-STD-188-141B. UHF standards, on the other hand, provide different challenges over the 225 to about 512 MHz frequency range, including short-haul line-of-sight (LOS) communication and satellite communications (SATCOM) and cable. This type of propagation can be obtained through different weather conditions, foliage and other obstacles making UHF SATCOM an indispensable communications medium for many agencies. Different directional antennas can be used to improve antenna gain and improve data rates on the transmit and receive links. This type of communication is typically governed in one example by MIL-STD-188-181B, which provides a family of constant and non-constant amplitude waveforms for use over satellite links, including calculation for higher transmit power and lower receive noise figures.
The joint tactical radio system (JTRS) has different designs that use oscillators, mixers, switchers, splitters, combiners and power amplifier devices to cover different frequency ranges. These modulation schemes used for these types of systems can occupy a fixed bandwidth channel at a fixed frequency spectrum. The systems usually include a memory of a coded waveform, such as a phase shift keying (PSK), amplitude shift keying (ASK), frequency shift keying (FSK), quadrature amplitude modulation (QAM), or continuous phase modulation (CPM) with a convolutional or other type of forward error correction code, for example, represented as a trellis structure. It should be understood that PSK, ASK, QAM and non-continuous FSK are memoryless modulations. For these modulations to have memory, Trellis-coded Modulation would need to be used for each specific M-PSK, M-QAM, M-ASK modulation type.
Throughout the communication industry, a requirement exists to improve power and spectral efficiency of a given modulation type, such as the PSK, ASK, FSK, CPM and QAM. A current industry standard uses filtering and other methods to eliminate any unnecessary out-of-band energy and improve the spectral efficiency and various forward error corrections (FEC) schemes to improve the power efficiency. Some limitations of these schemes are receiver demodulation complexity and non-constant amplitude.
Commonly assigned U.S. patent application Ser. No. 10/868,430, filed Jun. 15, 2004, and entitled, “Continuous Phase Modulation System and Method With Added Orthogonal Signals,” which is hereby incorporated by reference in its entirety, increases the distance property of coded, transmitted bits while taking advantage of any underlying memory (coding) scheme of those bits/symbols that are most likely to be received in error. That type of system is applicable to required modulation schemes that occupy a fixed bandwidth channel (LOS, cable or SATCOM) (fixed frequency spectrum). It takes advantage of the memory (usually represented as a trellis structure) of a coded waveform (i.e., TCM M-PSK, TCM M-ASK, continuous FSK, TCM M-QAM, or CPM with a convolutional (or other type) FEC code) by increasing the bandwidth of the signal and either improving the bit error rate performance or increasing the number of bits which can be encoded into a single symbol.
In that system, the addition of orthogonal or pseudo-orthogonal modulated waveforms (i.e., sin(ft/T), sin(2ft/T), PN spread sequences, etc.) increases the distance property of uncoded, transmitted bits while taking advantage of the underlying memory (coding) scheme of those bits/symbols which are most likely to be received in error. The amplitude modulated waveforms would provide discrimination to differentiate between different amplitude signals. The orthogonal or pseudo-orthogonal signals are added to the signal space of a modulation type to create a new, hybrid signal. The demodulation of this hybrid signal requires some modification to the receiver for optimal performance but does not effect the complexity of the original trellis decoder. Thus, the receiver complexity is not greatly increased. The addition of one orthogonal (or pseudo-orthogonal) waveform to a CPM-encoded signal is advantageous. This hybrid approach could be used with any modulation type which takes advantage of modulation or channel memory. In that system and method, a signal generator generates an encoded waveform, and a modulator adds at least one orthogonal or pseudo-orthogonal waveform to the trellis structure to create a non-constant envelope modulated signal that has increased bandwidth, improved bit error rate, or an increased number of bits encoded into a single signal.
This signal, however, has a non-constant envelope and it is desirable in some cases to add a signal and maintain a constant envelope signal.